In my main set, the 4iron has an extra stiff shaft and the rest of the clubs have regular shafts. I bought them used and that's how they came. I'm probably a regular to stiff flex player . .but definitely not extra stiff.
I don't notice any particular directional tendencies when I use my 4 iron - but it does feel really 'dead'.
I have another set of irons with all stiff shafts and I don't notice that I play my 4iron better in that set vs the one with the x-stiff shaft. It's really just feel . .or probably 90% feel. I could be losing a little distance but I'm not that consistent with my 4iron to be able to notice. It's definitely less than 10 yards if there's a difference.